Anti-tumor immunity of agammaglobulinemic birds is being studied. In vitro killing of tumor cells by both lymphocytes and macrophages from tumor-bearing agammaglobulinemic birds has been observed. Normal macrophages can also be activated to specifically kill tumor cells, but not embryo cells, by sensitized lymphocytes from agammaglobulinemic birds. Sera from normal birds with a progressing tumor are able to block both spleen cell and macrophage mediated killing or Rous sarcoma cells in vitro. However, sera from birds with a regressing tumor render normal macrophages cytotoxic for the specific tumor, but not embryo cells. Further studies are underway to characterize the lymphocyte-macrophage interactions and the effect of antibody and tumor antigen on this interaction.